The present invention relates to the art of diagnostic medical imaging. It finds particular application in conjunction with CT scanners, and will be described with particular reference thereto. However, it is to be appreciated that the present invention is also amenable to other like applications which employ backprojectors in image reconstruction from projection data (e.g., nuclear cameras).
Generally, CT scanners have a defined examination region or scan circle in which a patient, phantom, or other like subject being imaged is disposed. A beam of radiation is transmitted across the examination region from an x-ray source to oppositely disposed radiation detectors. The segment of the beam impinging on a sampled detector defines a ray extending from the source to the sampled detector. The source, or beam of radiation, is rotated around the examination region such that data from a multiplicity of rays crisscrossing the examination region are collected. At given angular source positions about the examination region, a sampled view or data line is collected which represents the projection data for that view.
The sampled data is typically convolved and backprojected into an image memory commonly described as a two-dimensional array or matrix of memory elements. Each memory element stores a CT number indicative of the transmission or attenuation of the rays attributable to a corresponding incremental element within the examination region. The data from each ray which crossed the incremental element of the examination region contributes to the corresponding CT number, i.e., the CT number for each memory element of the resultant image is the sum of contributions from the multiplicity of rays which passed through the corresponding incremental element of the examination region.
Commonly, the x-ray data is transformed into the image representation utilizing filtered backprojection. A family of rays is assembled into a view. Each view is filtered or convolved with a filter function and backprojected into an image memory. Various view geometries have been utilized in this process. In one example, each view is composed of the data corresponding to rays passing parallel to each other through the examination region, such as from a traverse and rotate-type scanner. In a rotating fan-beam-type scanner, each view is made up of concurrent samplings of the detectors which span the x-ray beam when the x-ray source is in a given position, i.e., a source fan view. Alternately, a detector fan view is formed from the rays received by a single detector as the x-ray source passes behind the examination region opposite the detector.
Various backprojection algorithms have been developed. For CT scanners, it is generally advantageous to have the quickest display of the resultant CT images. In many applications, the many millions of computations required renders general purpose computers inappropriately slow for backprojection. To obtain the image representations, the backprojections are normally performed with dedicated backprojection hardware. Examples of such backprojection processors are described in commonly assigned, U.S. patent application Ser. No. 09/056,563 to John Sidoti et al., and U.S. Pat. No. 5,008,822 to Brunnett et al., both incorporated herein by reference. However, certain limitations of the described backprojection processors make them inappropriate for certain applications. One such limitation is the cost associated with dedicated and/or customized specialty hardware which, in turn, requires customized programming.
Other methods have also been previously developed which describe backprojection using rendering techniques. However, these methods tend to use an additional accumulation buffer and only utilize single color channels. In addition, zoom reconstruction with such systems includes modifying the backprojection hardware parameters in order to backproject a subset of the projection data. The problem is, however, that commonly used rendering hardware of limited precision does not always support the precision required for backprojection when only a single color channel is utilized. Moreover, an additional hardware accumulation buffer, not commonly found in rendering hardware, is also required to maintain backprojection accuracy if only a single color channel is utilized. Zoom reconstruction on such backprojection systems requires resetting the backprojection parameters in order to interpolate on a subset of the projection data. These factors limit the ease of use and accuracy of such backprojection techniques which utilize single color channels.
The present invention contemplates a new and improved backprojector and backprojection technique which overcomes the above-referenced problems and others.